Method for manufacturing at least one area of a filter structure, in particular for a particulate filter in the exhaust gas system of an internal combustion engine

ABSTRACT

A method for manufacturing at least one area of a filter structure, in particular for a particulate filter in the exhaust gas system of an internal combustion engine, includes the following steps: a. manufacturing a mixture from a sintering metal powder and an organic binder; b. manufacturing a sheet from the mixture; c. structuring the sheet; and d. sintering.

FIELD OF THE INVENTION

The present invention relates to a method for manufacturing at least one area of a filter structure, in particular for a particulate filter in the exhaust gas system of an internal combustion engine.

BACKGROUND INFORMATION

Sintered metals are suitable for manufacturing metallic filter mats. In particular the processing of powdered metal is advantageous, because a well-defined density and pore size of the sintered metal may be adjusted via the particle size and particle size distribution of the powder used, as well as via the sintering conditions.

Currently, to manufacture filter mats made of sintered metal, a substrate material (e.g., a metallic mesh or metal fabric) is coated using sintering metal powder and sintered together. The substrate material or framework material has two functions: it helps to fix the sintering metal powder, and later stabilizes the filter pocket.

Such a filtering device is described in German Published Patent Application No. 101 28 936. The particulate filter shown in this patent is built into the exhaust gas system of a diesel engine. The filter walls in the known filter device are made of sintered metal and arranged in such a way as to form wedge-shaped filter pockets. The edges tapering into a tip of the filter pockets point against the direction of flow of the exhaust gas; the back narrow side of a filter pocket viewed in the direction of flow is open. The filter pockets are situated next to one another in such a way that a rotation-symmetric, annular filter structure is formed overall.

In the known particulate filter, the filter walls are formed by unstable sintered metal sheets or sintered metal mats, which are bonded to separate carrier or supporting structures, for example, perforated metal sheets, metal fabrics or the like.

Manufacturing a spreadable, mortar-like compound made of a sintering metal and a minimum amount of an organic binder is also known from the market. It is spread into the metal fabric or metal mesh by the bucket wheel principle. A smooth and thus relatively small filter surface is thus obtained.

Furthermore, a flowable paste or a slurry may be produced from sintering metal powder, an organic binder, and a solvent. The powder prepared in this way may then be applied either by immersing the metallic fabric or metallic mesh into the paste or slurry or the slurry may be cast onto or the paste may be imprinted on the metal fabric or mesh (by the silk-screen method, for example). In all variants of this process, a subsequent drying step is required in which the solvent is evaporated and the sintering metal powder is fixed on the metal framework.

Finally, it is known that sintering metal powder may be mixed with wax beads and this mixture may be blown onto the metallic mesh or metallic fabric. When heated, the melting wax beads cause the powder to adhere to the metallic framework.

Depending on the coating method used, there remains, however, the difficulty of accurately adjusting and controlling the coating thickness and coating density of the sintering metal layer. At the same time, a framework material considerably increases the weight and cost of a filter mat. It is therefore desirable to be able to omit a framework material, yet to produce defined and stable sintering metal sheets.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to refine a method of the type in such a way as to allow a filtering device having precisely defined characteristics to be manufactured in a cost-effective manner without using a supporting structure.

This object is achieved with a method of the type named in the preamble in that the method includes the following steps:

-   -   a. manufacturing a mixture from a sintering metal powder and an         organic binder;     -   b. manufacturing a sheet from the mixture;     -   c. structuring the sheet; and     -   d. sintering.

The use of a sheet has the advantage that its thickness, density, and the structuring of the sintering metal filling may be defined very precisely. The permeability of the sintered metal filter may be predefined precisely via these parameters.

In addition, such a sheet may be manufactured by a technically simple and cost-effective process with a reproducible quality. Continuous quality control and storage of such a sheet is also possible, which also facilitates the manufacturing process and lowers the manufacturing costs.

Structuring the sheet makes it possible to produce a defined surface structure and thus a controlled increase of the active filter surface area.

In a first refinement, the sheet is produced by sheet spreading, sheet casting, or sheet extrusion in step b. All above-named methods allow the sheet thickness to be precisely adjusted and a homogeneous, smooth, and air bubble-free sintered metal sheet to be manufactured.

It is also possible to produce multilayer green sheets from these individual sheets after the manufacture of the sheets in step b (green sheets) and before structuring.

Structuring takes place in step c, preferably at a temperature in the range of 80° to 150° Celsius, preferably in the range of 80° to 90° Celsius. The structuring temperature at which the sintered metal sheet is plastically deformable may be easily adjusted via an appropriate selection and amount of organic binder. The given temperature range is therefore particularly advantageous because the required energy input is limited and yet a satisfactory effect is achieved using customary organic and thermoplastic binders. This is true in particular for the range of 80° to 90° Celsius.

Structuring is preferably carried out with the help of embossing plates which are placed on and pressed into the sheet. However, a structured laminating roller may also be used or a plurality of rollers may be situated one behind the other. An arrangement of rollers on both sides is also possible.

In a further advantageous refinement of the method according to the present invention, both sides of the sheet are subjected to a structuring step. In this way, it is possible, for example, to obtain a double-sided waffle structure of the sheet, which has the advantage that not only a surface is produced, but also the mechanical stability of the sheet is improved via suitable geometries (waffle structures, honeycomb structures, see further below).

In a particularly advantageous embodiment of the method according to the present invention, a plurality of sheets, even sheets structured differently, may be laminated together in order to increase the layer thickness overall or locally. This may take place before or after the embossing step; however, initially a plurality of green sheets is advantageously laminated together in order to adjust the desired layer thickness or a desired gradient regarding powder size distribution and/or density distribution, and subsequently one surface or both surfaces is/are embossed. It is, however, also conceivable to laminate pre-structured, e.g., perforated, sheets together.

Furthermore, sheets containing more or less fine or course sintering metal powder may be combined to influence the pore structure of the finished filter sheets. Course and fine powders have different tendencies to bind during sintering. The green density of the sheets also has a great influence, i.e., it is important how tightly the particles are packed and how fast and how strongly they bind to one another. It is also possible to laminate two sheets together, one of which may have a particularly good structurability because of its composition, while the other is particularly stable mechanically, to thus make reliable handling in the production process possible. This is determined by the organic materials used during the sheet manufacture, i.e., the polymer binder, plasticizer additives, etc.

In another embodiment of the method according to the present invention, it is possible to also bond the sheet to a support structure. A metallic fabric, a mesh, or a perforated sheet is advantageously used as a support structure. These are cost-effective, cover only a small surface area, and thus allow high gas throughput during operation.

Optimum filter characteristics, in particular when using the filtering device as a particulate filter in the exhaust gas system of an internal combustion engine, are achieved when the sintering metal powder has a grain size of approximately 1 μm to 150 μm, preferably 40 μm to 70 μm, even more preferably 50 μm to 60 μm.

Favorable manufacturing costs are achieved if, in step a, the sintering metal powder having approximately 8% by weight of acrylate binder and butyl acetate as a solvent is processed to form a spreadable slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an internal combustion engine having a particulate filter having a filter structure.

FIG. 2 shows a perspective view of the filter structure of FIG. 1.

FIG. 3 shows two filter pockets of the filter structure of FIG. 1.

FIG. 4 shows a flow diagram of a method for manufacturing a filter wall of the filter structure of FIG. 1.

FIG. 5 shows a section of the sintering metal sheet before the structuring step.

FIG. 6 shows a section of the sintering metal sheet of FIG. 5 after structuring using a first embossing plate.

FIG. 7 shows a section of the sintering metal sheet of FIG. 5 after structuring using a second embossing plate.

FIG. 8 shows an embodiment of the structuring using waffle-shaped embossing plates.

FIG. 9 shows different embodiments of the multilayer green sheets according to the present invention.

DETAILED DESCRIPTION

An internal combustion engine is labeled with reference numeral 10 in FIG. 1. It has an exhaust system 12, in which a particulate filter 14 is situated. It is able to filter out soot particles, for example, from the exhaust gas of internal combustion engine 10. Particulate filter 14 has a housing 16 and a filter structure 18 situated in housing 16.

Filter structure 18 is shown in more detail in FIG. 2: It includes a plurality of wedge-shaped filter pockets 20, which are situated with their wedge edges tapering to a tip against the direction of flow of the exhaust gas. Filter pockets 20 are situated next to one another around a shared longitudinal axis, so that a completely rotation-symmetric filter structure 18 is formed. The radially inner and outer narrow sides of filter pockets 20 are closed. The narrow sides of filter pockets 20 situated in the back in the flow direction are open. The filter pockets are connected to one another in the area of their back ends in the direction of flow.

Two adjacent filter pockets 20 a and 20 b are depicted in FIG. 3. During operation, the exhaust gas enters an area between the two filter pockets 20 a and 20 b, passes through a lateral filter wall 22, and thus enters the inside of the particular filter pockets 20 a and 20 b. The exhaust gas flow is represented by an arrow 24. When passing through side wall 22, the particulates are separated from the exhaust gas and deposited on the upstream surface of side wall 22.

The walls, and in particular side walls 22 of filter pockets 20, are made of a porous sintered material. A method for manufacturing side walls 22 of filter pockets 20, for example, is shown in FIG. 4. First, sintering metal powder 26 having a grain size of approximately 50 μm to 60 μm, preferably 53 μm, is processed using approximately 2% to 8% by weight of acrylate binder 28 and a volatile organic solvent, for example, butyl acetate or alcohol 30 to a spreadable slurry 34 in a device 32. The slurry is processed into a 100 μm to 500 μm thick sintering metal sheet 38 (green sheet) using a film spreading device 36. If multilayer green sheets 39 are produced from these individual sheets, the thickness is preferably >450 μm.

This metal sheet or the multilayer green sheets are then structured with the aid of a device 40, either embossing plates or a structured laminating roller being able to be used here. The sintering sheets are heated to a temperature of approximately 80° C. for this purpose. Different embossing plates are placed onto and pressed in. The structure of the embossing plates shows clearly after being pressed onto the corresponding blank 42. Blank 42 is then sintered in a furnace 44, an almost single-piece composite 46 being obtained. Filter pockets 22 are subsequently manufactured from the sintered metal sheet by pressing, folding, and welding 48.

FIGS. 5, 6, and 7 show sections of a blank 42 used according to the present invention. FIG. 5 shows a blank 42 having a defined layer thickness and green thickness, which has been manufactured using a relatively thick sintered metal sheet 38. Surface 52 of sintered metal sheet 38 is smooth overall. Blank 42 shown in FIG. 6 corresponds to the blank manufactured using the method described in FIG. 4: It is apparent that the surface structure of embossing plate 40 is mapped on surface 52 of sintered metal sheet 38. FIG. 7 shows a blank 42, which has been manufactured with a differently structured embossing plate 40 and whose surface 52 has a pattern 54 embossed accordingly.

As mentioned previously, the mechanical stability of the sheet may be improved via suitable geometries. A sintered metal sheet having a large surface area and high mechanical stability may thus be manufactured, for example, via the method described below for manufacturing a “waffle iron structure.”

First, sintering metal powder 26 having a grain size of approximately 50 μm to 60 μm, preferably 53 μm, is processed by a device 32 with approximately 2% to 8% by weight of acrylate binder and a volatile organic solvent, for example, butyl acetate or alcohol 30 to form a spreadable slurry 34. The slurry is processed into a 100 μm to 500 μm, preferably >450 μm thick sintering metal green sheet 38 using a film spreading device 36. This sheet is then structured, for example, with the aid of embossing plates 56 shown in FIG. 8. For this purpose, the tool is heated to a temperature of approximately 80° C., green sheet 38 is placed in it, and the tool is closed under pressure. After opening the tool, green sheet 38 is removed again and now has the structure of the embossing plates. The blank is then sintered in a furnace 44, the geometry of three-dimensionally structured sheet 38 being fully preserved. Using this tool, it is also possible to use the sintering metal powder in other geometries (tablets, roughly sectioned plates, flatcake, etc.) instead of a defined sheet 38 in the form of a thermoplastic compound 58 (see FIG. 8).

The following example shows the manufacture of sintering metal sheets having a large surface area by using green sheets or multilayer sheets having a gradient.

First, sintering metal powder 26 having a grain size of 50 μm, for example, is processed with approximately 2% to 8% acrylate binder and a volatile organic solvent, for example, butyl acetate or alcohol 30 with the aid of a device 32 to form a spreadable slurry 34. This slurry is processed using a film spreader device 36 into a 100 μm to 500 μm, preferably <200 μm thick sintered metal green sheet 38. The same procedure is applied for powders, for example, of a grain size of 30 μm and 80 μm.

Other green sheet variants are sheet compositions having a higher proportion of organic material (e.g., 6% to 20% by weight of acrylic binder) or sheets having pore-forming additives, which evaporate completely during the sintering process. Sheets having different layer thicknesses also represent possible variants, as does the use of defined powder mixtures (narrow monomodal, bimodal, or trimodal grain size distributions) or different powder types (e.g., spherical, round powder types, irregular drop-shaped powder types, flake-type powders, etc.) in manufacturing the sheet.

These sheet variants 38 may be combined in different ways to form multilayer green sheets 39. The porosity and surface characteristics of sintered filter sheets 46 may thus be adjusted and defined in a controlled manner via suitable sintering profiles. Thus, for example, a filter sheet having large pores on one side and fine pores on the other side may be achieved by laminating different sheet variants after sintering, or a filter sheet having a high degree of surface roughness may be obtained after sintering when using sheets of different green densities, since the side having a lower green density shrinks in a non-homogeneous manner during sintering, forming porous surface structures. In both cases, sintered metal filters having a porosity gradient and a large surface area are obtained.

The above-mentioned multilayer green sheets 39, manufactured by laminating individual sheets 38 together, are processed as green sheets 38 and may, if needed, like sheets 38, be laminated onto a supporting framework such as a metallic mesh 62 and/or subjected to additional mechanical structuring by embossing.

FIG. 9 shows different design options of multilayer green sheets 39. FIG. 9A shows a triple laminate 60 made of three green sheets 38 of the same type with metallic mesh 62 as the supporting structure applied by lamination. FIG. 9B shows a double laminate 64 made of two different green sheets 38, one having a fine powder and the other having a coarse powder (66, 68). The surface of the sheet having a coarse powder 68 is additionally structured by embossing to increase the filtering surface area. FIG. 9C shows a green sheet having a “waffle structure,” manufactured using a multilayer green sheet 39 having three layers (70, 72, 74) each with different powder filling, and finally FIG. 9D shows a double laminate 76 made of two different green sheets 78, 80, one sheet being highly filled, and the other having a low degree of filling with metal powder, and also possibly containing pore-forming additives. FIG. 9D again shows a metallic mesh 62 applied by lamination. 

1-14. (canceled)
 15. A method for manufacturing at least one area of a filter structure in an exhaust gas system of an internal combustion engine, comprising: manufacturing a mixture from a sintering metal powder and an organic binder; manufacturing a sheet from the mixture; structuring the sheet; and performing a sintering operation.
 16. The method as recited in claim 15, further comprising: before the structuring step, manufacturing multilayer green sheets from the sheet manufactured; and structuring the multilayer green sheets.
 17. The method as recited in claim 15, wherein the sheet and/or the multilayer green sheet have a sintering metal powder size distribution gradient and/or density distribution gradient.
 18. The method as recited in claim 15, wherein the sheet is produced by sheet spreading, sheet casting, or sheet extrusion.
 19. The method as recited in claim 15, wherein the structuring takes place at a temperature in the range of 80° C. to 150° C., preferably in the range of 80° C. to 90° C.
 20. The method as recited in claim 15, wherein the structuring is achieved by embossing.
 21. The method as recited in claim 20, wherein embossing plates and/or structured laminating rollers whose surface structure is mapped on the sheet or the multilayer green sheet are used for embossing.
 22. The method as recited in claim 15, wherein a sintering metal powder having a grain size of approximately 1 μm to 150 μm, preferably 40 μm to 70 μm, more preferably 50 μm to 60 μm, is used.
 23. The method as recited in claim 15, wherein the sintering metal powder is processed in step a with approximately 2% to 8% by weight of acrylate binder and a volatile organic solvent to form a spreadable slurry.
 24. The method as recited in claim 23, wherein the volatile organic solvent is butyl acetate or alcohol.
 25. The method as recited in claim 15, wherein the sheet or the multilayer green sheet is structured on both sides.
 26. The method as recited in claim 25, wherein the sheet or the multilayer green sheet is given a double-sided waffle structure.
 27. The method as recited in claim 15, wherein the sheet or the multilayer green sheet is additionally bonded to a supporting structure.
 28. The method as recited in claim 27, wherein a metallic fabric, a metallic mesh, or a perforated sheet is used as the supporting structure. 